Reference for Chullin 58:27

אמר רבא

Now if you say that the term shechitah applies to the entire process of slaughtering from beginning to end, then the Tanna should have drawn a distinction even in the slaughtering; thus: If any invalidating defect befell it during the slaughtering, it renders unclean the clothes worn by those who took part in any service before the defect,<span class="x" onmousemove="('comment',' For up to the moment of the occurrence of the defect there was a valid shechitah, since this term, according to R. Johanan, applies even to the first stage of the slaughtering, so that the Red Cow should render unclean the clothes worn by those who took part in any service before the occurrence of the defect.');">¹³</span>

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